Problem: test_large_msg kills my system temporarily

And I'm on a reasonably sized laptop. I think allocating INT_MAX
memory is dangerous in a test case.

Solution: expose this as a context option. I've used ZMQ_MAX_MSGSZ
and documented it and implemented the API. However I don't know how
to get the parent context for a socket, so the code in zmq.cpp is
still unfinished.

src/ctx.hpp

@@ -199,6 +199,9 @@ namespace zmq
         //  Maximum number of sockets that can be opened at the same time.
         int max_sockets;
 
+        //  Maximum allowed message size
+        int max_msgsz;
+
         //  Number of I/O threads to launch.
         int io_thread_count;